feat(swift): add Swift highlighting, sourcekit-lsp and formatting

LazyVim has no Swift extra, so configure the pieces swift.org's Neovim
guide uses: the Treesitter swift parser and sourcekit-lsp from the
installed toolchain rather than Mason. lspconfig also assigns
sourcekit C, C++ and Objective-C, which would put a second server
beside clangd on those files, so limit it to Swift.

Format on save with conform's swift formatter, which runs the
toolchain's `swift format`; the standalone swift-format binary isn't
on PATH.

lua/plugins/swift.lua

@@ -0,0 +1,29 @@
+return {
+  {
+    "nvim-treesitter/nvim-treesitter",
+    opts = { ensure_installed = { "swift" } },
+  },
+  {
+    "neovim/nvim-lspconfig",
+    opts = {
+      servers = {
+        sourcekit = {
+          -- sourcekit-lsp ships with Xcode and the Swift toolchain, not Mason.
+          mason = false,
+          -- lspconfig also lists C, C++ and Objective-C, which clangd handles.
+          filetypes = { "swift" },
+        },
+      },
+    },
+  },
+  {
+    "stevearc/conform.nvim",
+    opts = {
+      formatters_by_ft = {
+        -- Runs `swift format` from the toolchain; the standalone
+        -- swift-format binary isn't on PATH.
+        swift = { "swift" },
+      },
+    },
+  },
+}